The invention relates to a method of manufacturing an electroluminescent display screen, in which a first layer of an electrode material is deposited on a flat substrate, in which layer a first pattern of electrodes is formed, whereafter said first pattern is provided with an organic electroluminescent material and a second pattern of electrodes, using a mask with strips of a masking material, which strips are separated from each other by spaces corresponding to the second pattern, in which spaces the electrodes of the second pattern are formed by deposition of a second electrode material. The invention also relates to a device for carrying out this method.
At the location where electrode patterns cross each other, pixels are formed which emit light if a suitable electric voltage is applied to such an electrode crossing. The display screen may include very many pixels. The pixels may be, for example, segments of digits, letters and other characters to be displayed. The pixels may also form a matrix. Segmented screens are designed for a specific application, for example as a screen for a digital clock. Matrix screens find a much wider application, and can be used, for example, for displaying TV images. However, driving matrix screens is more complex than driving segmented screens.
Customarily, the substrate and the first electrode pattern are transparent, so that the light can egress on the substrate side of the display screen. On the substrate, a single display screen may be formed, such as a screen for displaying TV images, but also a number of display screens, such as the much smaller, segmented display screens used in digital clocks.
Organic electroluminescent materials are permeable to water. After deposition of the electroluminescent material, the subsequent process steps must be carried out in a dry manner. If not, water can penetrate as far as the boundary between the second electrode material and the electroluminescent material. As a result, the injection of charge carriers into the electroluminescent material is hampered and hence a satisfactory operation of the display screen prevented. As a result, customary photolithographic processes cannot be applied.
EP-A-732868 discloses a method of the type mentioned in the opening paragraph, in which, after the first electrode pattern has been formed, the mask is formed thereon in a customary manner by means of photolithography. The assembly is dried, whereafter a layer of an organic electroluminescent material and the second layer of electrode material are successively vapour-deposited on the strips of masking material and in the spaces between these strips. The strips of masking material are so high that a second pattern of separated electrodes is formed. The strips of masking material form part of the display screen and serve as an electrical insulation between the electrodes.
To fill the spaces between the strips of masking material with the organic electroluminescent material, said material is provided by vapor deposition. Vapor deposition is a process which can suitably be used to fill the spaces; simpler and cheaper techniques, such as spin-coating and screen-printing cannot be used. If, in the known method, the layer of electroluminescent material is deposited by a process other than vapor deposition, it cannot be guaranteed that the layer has a uniform thickness in all spaces. As a result, in operation, the pixels will not all emit the same amount of light. Due to the limited deposition possibilities, the number of organic electroluminescent materials which can suitably be used in the known method is limited.